32 THE SKATE 



this bending, which results in an s-shaped structure, the 

 ventricle is brought into contact with and overlies the 

 sinus venosus, and the auricle is brought into contact with 

 and is beneath the conus arteriosus. 



First note the distribution of the posterior coronary 

 artery coming from the posterior corners of the pericardial 

 cavity. Their origin will be indicated later. 



Kemove the heart from the pericardial cavity by cutting 

 it at the base of the ventral aorta and freeing the sinus 

 venosus from the transverse septum. Split open the sinus 

 venosus which leads into the auricle by a sino-auriculax 

 aperture which is supplied with sino-auricular valves. 

 Open the auricle and wash out the blood clot. It opens 

 into the ventricle through the auriculo-ventricular aperture 

 which is supplied with auriculo-ventricular valves. Open 

 the ventricle to locate these valves. Within the ventricle 

 note the numerous cavities and crevices in the spongy inner 

 wall where the blood is held, and the muscular band, the 

 columnae oarnae. Open the conus arteriosus which is 

 provided with many pocket-like semi-lunar valves. There 

 are three longitudinal rows of valves and five valves in each 

 of the rows. To locate and open these pockets run the 

 point of a probe from the anterior end towards the pos- 

 terior end. 



In the skate, as in all fishes, there is a single circulation 

 through the heart. The heart contains only venous blood, 

 which enters the sinus venosus from the systemic veins and 

 passes through the auricle, ventricle, and conus arteriosus 

 which connects with the ventral aorta, from which the blood 

 is distributed to the gills by means of the afferent branchial 

 vessels. The blood passes through the gill filaments and 

 is aerated. Since the heart contains no aerated blood its 

 walls are supplied by the coronary arteries from the arterial 

 system. 



